This invention relates to ball bearings and the like for use in mechanical structures including aircraft, and more particularly to a ceramic ball bearing assembly that provides ceramic-on-ceramic operation.
Current manufacturing practices used in the manufacture of ball bearings result in difficulties associated with allowing 100% of the contact surfaces to be ceramic material. This limitation is driven by the difficulty to mount low tensile strength, high Young""s modulus, low thermal expansion rate ceramic rings on to steel shafting as the corresponding characteristics of the steel shafting are incompatible with those of ceramic rings. Ceramic rings are sensitive to tensile stresses which can result in destructive overload failure. For high DN applications and for application across large temperature regimes, the current state of the art is to use ceramic balls with steel raceway rings. Ceramic balls have lower adhesive tendencies than all-steel balls and high hot hardness. The achievement of a robust all-ceramic bearing would result in an operating advantage over partially, or hybrid, ceramic bearings in high-speed, high temperature low lubrication, and/or hot environments.
Consequently, it would be of some advantage to provide a robust means by which 100% of the contact surfaces in a ball bearing or the like could be ceramic. As indicated above, mounting ceramic rings directly on steel shafting tend to destroy the ceramic rings due to operating incompatibilities. Consequently, it would be an advance in the art to provide ceramic ball bearings and assemblies thereof that were compatible with steel drive shafts, other steel shafts, and the like.
The present invention provides low-cost ceramic ring/steel ring assemblies that can be achieved through the application of braze technology, creating residual compressive loading of the ceramic ring with the high load capacity steel. A steel ring circumscribing the outer diameter of a ceramic outer raceway will shrink more while cooling down from braze temperature than the ceramic ring in the assembly. This results in residual compression in the steel-rim hoop structure in both hoop and axial directions in the ceramic ring. Ceramics are generally materials that withstand compression well but may suffer destructively if subject to tension or tensile forces. Consequently, by using the steel ring""s material characteristics to create residual compression upon the ceramic ring, the ceramic ring can be protected from destructive tensile forces.
For an inner ceramic ring or raceway, a steel ring on the inside of a ceramic inner raceway shrinks more during cool down from braze temperature than the circumscribing ceramic ring. Therefore suitable means are required for managing, the residual compression in both hoop and axial directions in the ceramic ring.
Residual compressive stress, when properly managed, can serve to protect the weak-in-tension but strong-in-compression ceramic rings from cracking during operation, as well as during a press-fit mounting of the ceramic bearing assembly. A bearing which properly manages the ceramic residual stresses enables a ceramic ball bearing with 100% of the contact surfaces being ceramic materials to be achieved as opposed to the current state-of-the-art hybrid system with a ceramic ball and steel races. Braze technology set forth herein also enables the use of segmented ceramic ring segments if required due to dissimilar thermal expansion, centrifugal loading, or other operating characteristics.
In an alternative embodiment of the present invention, an inverse hybrid configuration may be achieved where a steel ball bearing runs against the ceramic raceway. The technical advantage to this embodiment is that a difficult-to-machine steel material can be used for the rolling ball using existing mass production techniques which reduce the cost for such steel balls. In conjunction with ceramic raceways, the ceramic tribological (low-wear, low-lubrication requirements) contact advantages are maintained by the ceramic rings. The contact stresses are reduced by the use of steel balls having a low Young""s modulus. Such steel balls may be made of GB42, Cobalt alloys, or be hollow.
In a third alternative embodiment, AES (Aircraft Engine System) high temperature unlubricated valve bearings may be achieved. Low-cost, high-temperature ceramic ring/steel ring assemblies can be achieved through the application of high-temperature braze technology to create residual compressive loading of the ceramic ring coupled with a high load capacity steel outer or inner ring. This enables a true ceramic ball bearing as opposed to current state-of-the-art ceramic hybrid bearings with ceramic balls turning in steel races. Such ceramic ball bearings set forth herein offer increased wear and erosion resistance, which is a significant problem in the field. Such ceramic ball bearings have the potential to be significantly advantageous for AES high temperature valves.
Other features and advantages of the present invention will become apparent from the following description of the preferred embodiment(s), taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.